Secrets Of The Masters: Core Java Job Interview Questions

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Drool, Britannia? Is the UK Failing the Cloud?

By Roger Strukhoff

Richard Davies wrote: The UK has a good crop of technology pioneers in cloud computing - for example ElasticHosts, FlexiScale, Flexiant, OnApp - and also some strong government initiatives such as G-Cloud. We will have to see whether this kind of technical leadership converts into swift mass-market adoption or not.

Jan. 8, 2012 11:38 AM EST

30 Java Interview Questions

* Q1. How could Java classes direct program messages to the system console, but error messages, say to a file?

A. The class System has a variable out that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This how the standard output could be re-directed:

Stream st = new Stream(new FileOutputStream("output.txt")); System.setErr(st); System.setOut(st);

* Q2. What's the difference between an interface and an abstract class?

A. An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.

* Q3. Why would you use a synchronized block vs. synchronized method?

A. Synchronized blocks place locks for shorter periods than synchronized methods.

* Q4. Explain the usage of the keyword transient?

A. This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).

* Q5. How can you force garbage collection?

A. You can't force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.

* Q6. How do you know if an explicit object casting is needed?

A. If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example:

Object a; Customer b; b = (Customer) a;

When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.

* Q7. What's the difference between the methods sleep() and wait()

A. The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.

* Q8. Can you write a Java class that could be used both as an applet as well as an application?

A. Yes. Add a main() method to the applet.

* Q9. What's the difference between constructors and other methods?

A. Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.

* Q10. Can you call one constructor from another if a class has multiple constructors

A. Yes. Use this() syntax.

* Q11. Explain the usage of Java packages.

A. This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.

* Q12. If a class is located in a package, what do you need to change in the OS environment to be able to use it?

A. You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.java. In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:

c:\>java com.xyz.hr.Employee

* Q13. What's the difference between J2SDK 1.5 and J2SDK 5.0?

A.There's no difference, Sun Microsystems just re-branded this version.

* Q14. What would you use to compare two String variables - the operator == or the method equals()?

A. I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object.

* Q15. Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?

A. Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.

* Q16. Can an inner class declared inside of a method access local variables of this method?

A. It's possible if these variables are final.

* Q17. What can go wrong if you replace && with & in the following code:

String a=null; if (a!=null && a.length()>10) {...}

A. A single ampersand here would lead to a NullPointerException.

* Q18. What's the main difference between a Vector and an ArrayList

A. Java Vector class is internally synchronized and ArrayList is not.

* Q19. When should the method invokeLater()be used?

A. This method is used to ensure that Swing components are updated through the event-dispatching thread.

* Q20. How can a subclass call a method or a constructor defined in a superclass?

A. Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor.

Questions for senior developer job positions follow on the next page...

Published Jun. 17, 2007— Reads 976,221 — Feedback 176
Copyright © 2007 SYS-CON Media, Inc. — All Rights Reserved.
Syndicated stories and blog feeds, all rights reserved by the author.

▪ Your First Java Program

▪ Intro to Object-Oriented Programming with Java

▪ Methods, Constructors, Overloading and Access Levels

▪ Java Exceptions

▪ Java Streams Basics

▪ Reading Data from the Internet

▪ Java Serialization - Lesson 7

▪ Java Serialization

▪ Teaching Kids Programming: Even Younger Kids Can Learn Java

▪ Taking a Sun Java Studio Creator for a Drive

▪ Java Basics: Introduction to Java Threads, Part 1

▪ Interviewing Enterprise Java Developers

▪ Java Gotchas: Instance Variables Hiding

▪ Java 5.0 - The "Tiger" Is Out of Its Cage

▪ Grids, Blades, and Java - Wall Street's Top Technologies

▪ Java Basics: Introduction to Java Threads, Part 2

▪ i-Technology Opinion: Outsourcing...to Students

▪ SYS-CON Webcast: Eclipse IDE for Students, Useful Eclipse Tips & Tricks

▪ Our JUGs Need a Push-Up

▪ Java Basics: Lesson 11, Java Packages and Imports (Live Video Education)

▪ Another Brick in the Wall

Related Links

▪ Java 2 Enterprise Edition 1.4

▪ Java Programming for Kids

▪ Java Tutorial for the Real World

About Yakov Fain
Yakov Fain is a Managing Director of Farata Systems, consulting, training and product company. He has authored several Java books, dozens of technical articles. SYS-CON Books released his latest co-authored book , Rich Internet Applications with Adobe Flex and Java: Secrets of the Masters in Spring 2007. Sun Microsystems has nominated and awarded Yakov with the title Java Champion. He leads the Princeton Java Users Group. He is an Adobe Certified Flex Instructor. Yakov co-athored the O'Reilly book "Enterprise Application Development with Flex". He twits at twitter.com/yfain.

Reader Feedback: Page 1 of 12

#176	Raghu commented on 9 Jul 2008 A message to all,interview questions given here can be generic ONLY.Its not like for 2+ years,4+ years etc.It purely depends on the company and the role for which you are interviewed.Some of the interview questions i faced recently.I was able to clear all the theoretically related so they asked me by giving a scenario. 1. You need to read all the details from say.. www.yahoo.com/index.html, save it in a file in yor local PC. What API will you use in Java and what method?? 2. You are saving a set of employee names and their salaries in a HashMap like that.. How will you sort them in descending order based on the salary? This can be done by implementing the comparator interface. If you need further information,drop a mail 2 graghuu@gmail.com Cheers, Raghu
#175	aruna commented on 20 Jun 2008 Hi, Could you please send me the interview questions in java,jsp,struts,jdbc,corejava for 1+ yers of experience as software engineer. Thanks you,
#174	Ashvini commented on 16 Jun 2008 Hi, Could you please send me the interview questions in java,jsp,struts,jdbc,corejava for 2+ yers of experience as software engineer. Thanks you, Ashvini. S
#173	. commented on 13 Jun 2008 these questions are stupid these are moronic questions that idiots that don't know how to interview asks lets get to some real world questions
#172	deepa commented on 11 Jun 2008 Please send me expert level questions on jdbc 4.0
#171	sandeep commented on 6 Jun 2008 try http://www.java-interview.com for all the java related interview questions
#170	shikha commented on 30 May 2008 please send me the interview questions in java,jsp,jdbc,corejava for 1+ yers of experience as a software engineer. thanks you,
#169	sreekanth commented on 28 May 2008 Thank you very much for such a good article.....as I'm in search of job this article would certainly be helpful for my preparation.
#168	sandeep commented on 20 Apr 2008 please send me the interview questions in java,jsp,jdbc,corejava for 1+ yers of experience as a software engineer. thanks you, sandeep
#167	siddharth commented on 18 Apr 2008 I have 1+ yrs experience of working in java.. please help me by sending the interview questions..
#166	varun commented on 5 Apr 2008 Data Migration/Conversion: It can be used to convert sql server tables into mysql tables
#165	rajyalakshmi.M commented on 26 Mar 2008 I need the type of questions asked in an interview for a job in java/j2ee foe 2+ years experience Thanks
#164	Ranjani commented on 12 Mar 2008 I need the type of questions asked in an interview for a job in java/j2ee with about 2 years experience. Thanks
#163	archana commented on 7 Mar 2008 Hi, Could you please send me the interview questions in java,jsp,struts,jdbc,corejava for 2+ yers of experience as software engineer. Thanks you, Archana.
#162	sony commented on 1 Feb 2008 I have acquired abt 4-5yrs of Java/J2EE exp and will be attending an interview in the States...Could you pls let me know on what are the tips that I need and pls help me with some questions that they will ask me?